Magnesium oxide was found to show activity for the reduction of carbon dioxide to carbon monoxide under photoirradiation using hydrogen as a reductant. Fourier transform infrared spectroscopy was applied to a study of the reaction mechanism by detection and identification of the surface species arising during the photoreaction. The formation of surface formate ion was observed during the photoreaction. Since CO was produced from the surface formate in the presence of CO2 under irradiation, the surface formate was a reaction intermediate which acted as a reductant and converted another CO2 molecule to CO. The correlation of the reaction activity with the amount of introduced CO2 indicated that adsorbed carbonate was reduced by H-2 to the surface formate, and that the surface formate also reduced the adsorbed carbonate to CO. The IR spectra showed the difference in the adsorption form of CO2 between the adsorbed carbonate reduced by H-2 to the surface formate and that reduced by the surface formate to CO.